A neutron therapy facility is currently being organized at the Fermi National Accelerator Laboratory, Batavia, Illinois (see "parent application" entitled "Fermilab Nucleon Therapy Facility"). This will involve the use of high-LET neutrons generated from protons of peak energy 67 MeV impinging on a lithium target. A series of in vitro and in vivo radiobiological measurements is planned as part of the preceding application. To facilitate the start of clinical studies, however, it is essential that certain in vitro measurements be undertaken ahead of time. Therefore, the aim of this research is to characterize sufficiently the radiobiological properties of the neutron beam to permit initial clinical studies to be started. More comprehensive radiobiological measurements, in vitro and in vivo, are to follow as called for in the parent application. The initial radiobiological experiments are to be confined to cultured mammalian cells (V79 Chinese hamster and possibly HeLa and other cells). The neutron spectrum anticipated is expected to be quite different from those available from currently existing neutron therapy facilities. For this reason alone, it is essential that those radiobiological characteristics of the beam which relate directly to its therapeutic potential be worked out ahead of time. These will include the relative biological effectiveness and the oxygen enhancement ratio of single and fractionated doses relative to cell survival. Other end points are to be included if possible. From these studies, an assessment will be made by the therapists in charge of the Nucleon Therapy Facility of how closely the radiobiological properties of the Fermi Lab neutron beam conform to predictions based upon the properties of other neutron sources. It is expected that upon the completion of this assessment, initial clinical studies can be started with an adequate measure of confidence. Although the physical setup for the work will not be optimal at the start, since the experiments are not intended to break new ground it is expected that they can be completed in a relatively short time.